In recent years, there is a demand for digital multifunction printers, digital cameras, and the like, attaining high speed and high color saturation, and therefore, a large number of digital signals are required to be transmitted at high speed. Accordingly, serial transmission capable of transmitting a large amount of data at high speed using a small number of transmission lines has been widely used.
In a serial transmission method, low-speed parallel signals such as data, addresses, and control lines are serialized and output to a transmission line as differential signals and the transmitted serial signals are deserialized on a reception side so that parallel signals are obtained. A clock signal is embedded in a row of the serialized data to be transmitted, and the clock signal and the data are reproduced on the reception side.
When a high-speed signal is transmitted to a long lossy transmission line such as a cable, a portion of a component of the signal may be emitted from the cable serving as an antenna, and as a result, operations of other apparatuses may be affected. Therefore, electromagnetic interference (EMI) by the apparatus should be controlled.
In a clock-embedded type serial transmission, data and a synchronous clock are simultaneously serialized and data which is coded such that a logical rate of transition between a high level and a low level becomes 50 percent is transmitted. Therefore, a low level or a high level of a transmitted serial signal does not consecutively appear in a plurality of bits but repetition waveforms in a basic cycle of one bit mainly appear. Accordingly, strong EMI from a serial transmission system is observed at timings of integral multiples of one-bit cycle of the serial signal. Furthermore, a spectrum of the serial signal to be transmitted by a rectangular wave is represented by a sinc function, and it is known that a frequency of an integral multiple of one-bit cycle does not have a spectrum. Specifically, the EMI is generated in frequencies which do not include a spectrum of a transmission signal.
To suppress the generation of the EMI, a band elimination filter or a notch filter which includes a lumped parameter circuit including lumped parameter elements such as coil elements or capacitor elements may be used. In this case, since the filter is constituted by the lumped parameter element, a component element value is considerably small in a high frequency region such as a GHz band, and accordingly, it is difficult to obtain a desired cutoff frequency when a standard component is used.
Here, PTL1 discloses a band elimination filter constituted by a distributed constant circuit. In this case, an open stub having an electrical length of ¼ wavelength relative to a fundamental frequency is connected to a line (power supply line) connected to an LSI serving as a noise source so as to attenuate noise of frequencies of odd multiples of the fundamental frequency. Furthermore, to attenuate frequencies of even multiples of the fundamental frequency, e.g., a frequency of twice the fundamental frequency, PTL 1 discloses additional connection of an open stub of ¼ wavelength for the frequency of twice the fundamental frequency.